Photolithographic fabrication of state of the art microelectronic integrated circuits requires high resolution (one micron or even less) imaging, together with relatively little overlay error between successive masks. To that end, modern photolithographic exposure tools employ high numerical aperture imaging optics which enable them to achieve sub-micron resolutions, even though that causes them to have a very shallow depths of focus. Therefore, the focus and overlay error of such equipment are checked regularly, so that prompt corrective action may taken to compensate for variations in the focus, overlay alignment, magnification or similar operating parameters of the exposure tool.
Heretofore, the characterization and optimization of the imagery of photolithographic exposure tools typically have depended upon using the exposure tool to pattern a resist coated "dummy" wafer and then developing the resist to enable an operator to examine the pattern with an optical microscope and/or an electrical probe. Optimum focus conventionally is determined by a subjective examination of a focus/exposure matrix formed on the dummy wafer, even though the individual resolutions of the several resist patterns within such a matrix not only depend upon the actual optical resolution of the exposure tool, but also upon the uniformity of the resist processing across the wafer, the resolution of the resist, and the vibration of the image. Similarly, overlay errors normally are determined by comparing optical vernier patterns having finite resolutions down to 0.1 micron or so. Thus, even experienced, highly skilled operators can misinterpret the test data and make erroneous and potentially costly changes to the set-up of the exposure tool as a result of errors in judgement.
Techniques have been developed for evaluating the resolution and overlay alignment of photolithographic exposure tools in situ, thereby eliminating the expose/develop/examine cycle which causes the above-described set-up process to be so time consuming and labor intensive. See, for example, Brunner, T. A. and Smith, S. D., "Moire Technique for Overlay Metrology," SPIE Proceedings, Vol 480, May 1984, pp. 164-170. Briefly, it has been shown that the phase and contrast of moire fringes between a grating mask and a grating wafer mounted on an photolithographic exposure tool relate to the overlay error and resolution, respectively, of the exposure tool. Accordingly, an array of photodetectors have been provided for reading out such moire fringes on a pixel-by-pixels basis. However, a more straightforward in situ resolution and overlay alignment evaluation technique still is needed.